A number of approaches have been used in arriving at the nutritional requirements of an individual and the RDA for a population. The general principles are:
Dietary intakes: This approach has been used in arriving at the energy requirements of children. Energy intakes of normally growing children are utilized for this purpose.
Growth:The requirement of any particular nutrient or the breast milk intake, for satisfactory growth has been utilized for defining requirements in an early infancy.
Nutrient balance:The minimum intake of a nutrient for equilibrium (intake = output) in adults, and nutrient retention consistent with satisfactory growth in children, have been used widely for arriving at the protein requirements.
Obligatory loss of nutrients:The minimal loss of any nutrient or of its metabolic products (viz., nitrogenous end products in the case of proteins) through normal routes of elimination, viz., urine, faeces and sweat, is determined on a diet devoid of or very low in the nutrient (for example, a protein free diet). This information is used to determine the amount of nutrient to be consumed daily, through diet, to replace the obligatory loss. In infants and children, growth requirements are added to the above maintenance requirement.
Factorial approach:In this approach, the requirements for different functions are assessed separately, and added up to arrive at the total daily requirement. This is the basis of arriving at energy requirements.
Nutrient turnover:Data from turnover of nutrients in healthy persons, using isotopically labeled nutrients have been employed in arriving at requirements of certain nutrients. Requirements of Vitamin A, Vitamin C, iron and Vitamin B12, have been determined on this basis. Earlier, radio isotopic labelled compounds were utilized and currently compounds labelled with a stable isotope are being increasingly used to determine the turnover of nutrients in the body. Stable isotopes are particularly useful for infants, children and women during pregnancy and lactation where radioisotopes are contra indicated.
Depletion and repletion studies: This approach has been employed in arriving at the requirement of water-soluble vitamins. The levels of a vitamin or its coenzyme in serum or tissue (erythrocytes, leucocytes) are used as a biochemical marker of the vitamin status. Requirements of ascorbic acid, thiamine, riboflavin, niacin and pyridoxine have been established employing this approach. The subjects are first fed a diet very low in the nutrient, under study, till the biochemical parameters reach a low level after which the response to feeding graded doses of the nutrient is determined. The level at which response increases rapidly is an indication of requirement.
The Recommended Dietary Allowances (RDA) for various nutrients for different age groups (ICMR 2010) (Table).